Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KLOT 190024

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
724 PM CDT Tue Jul 18 2017

218 PM CDT

Through Wednesday...

For the remainder of the afternoon and through this evening,
increasingly warmer, more humid conditions are setting up as high
pressure over the region begins to slide to the east and prevailing
southerly to southeasterly flow sets up in the lower levels.  The
main forecast concern this afternoon will be the inland progression
of a lake breeze boundary.  Winds have turned onshore along Lake
Michigan, with cooler air spreading inland.  latest high res
guidance suggests that the lake breeze may reach inland as far as
O`Hare and Midway and has already pushed through Waukegan.
Temperatures this afternoon should top out in the upper 80s, with a
few locations possibly pegging the 90 F mark.  With the lake breeze
pushing inland, the lake front will only see temps in the middle to
upper 70s.

The next concern will be timing of chances of showers and
thunderstorms late tonight.  This activity should be along and in
advance of a cold front, which extends from western Lake Superior
through the upper Missouri Valley.  The longer range guidance is
relatively consistent in slowly pushing the front southward
overnight, reaching the Rockford area by around midnight and then
continuing south and east, pushing south of the CWA durg the early
morning hours.  There should be a fairly widespread area of showers
and thunderstorms in advance of the front, in a zone of strong
warm/moist advection, but the front will be pushing through nrn IL
and into nrn Indiana durg the overnight to early morning hours,
which would be a diurnally unfavorable time for frontal convection.
However, while confidence in timing and coverage of thunderstorms is
relatively low, currently feel that there will likely be at least
scattered showers and thunderstorms overnight.  The front should
stall out across the CWA, and then lift back north again as low
pressure deepens over the northern plains.  With upper ridging
building across the upper midwest tomorrow, temperatures will likely
be at least a couple degrees higher, with a more widespread area see
90+ degrees.  Weak onshore flow persisting through the day should
keep the lakefront cooler, with highs likely only reaching the upper
70s along the IL lakefront and low 80s along the nrn IN lakefront.
With sfc dewpoints expected to climb into the lower 70s, heat index
reading may reach or exceed 100 F, especially for locations south of
I-80, where temps and dewpoints are likely to be highest.


311 PM CDT

Wednesday night through Tuesday...

Main forecast concerns/challenges are with periodic thunderstorm
chances beginning from the start of the period into the weekend,
with the possibility of severe weather, heavy rainfall and
flooding. Hot and humid conditions will also be possible by the
end of the work week into the start of the weekend.

Confidence continues to be on the lower side with the exact
details, however, overall setup is going to favor periodic
thunderstorm development across the region. Ridging across the
central CONUS will provide a setup for periodic waves of energy to
stream across the region into the weekend. Also, instability and
moisture axis shift back overhead, with strong instability and
high PWATS of around 2 inches a definitely possibility. Timing and
placement of this possible forcing will be key with gaining a
better understanding of where the thunderstorm complexes will
track. Guidance continuing to focus in on another mid level wave
moving throughout the region late Wednesday. MCS development would
be a likely scenario from this forcing, with any MCS then
progressively moving through the region. Guidance still varying on
exactly where this will track with some guidance keeping the
complex of storms to the north, while other guidance pushing it
into northern Illinois by early Thursday morning. Once again
instability/moisture axis is forecast to be situated right into
northern Illinois. This setup would support a more southerly
track, with the possibility for any storms to move through the
instability axis into the CWA. So, am thinking these thunderstorms
will have decent chance of getting into northern Illinois and
northwest Indiana, but still have lower confidence with exact
placement. This is the first period to keep note of as severe
weather and heavy rainfall will be possible. This will be
especially troublesome in terms of the heavy rainfall and possible
flooding, if this were to track through areas north of I-88 where
very wet conditions are in place and flooding/elevated rivers are
already occurring. A warm day expected Thursday but convection
trends will control the extent of this warmth. Air mass will be
supportive of highs around 90 and with dewpoints likely reaching
well into the 70s, heat indices above 100 still appear possible
particularly across the southern CWA.

MCS development appears possible once again Thursday night into
Friday, with the CWA remaining in the general track of any
approaching storms. Still lower confidence with exact trends, but
still another period to monitor as setup would favor additional
thunderstorms with heavy rainfall, and possible severe weather.
Despite this lower confidence, several synoptic features in place
that would support wetter conditions over dry conditions. No real
big change in the pattern into the start of the weekend, with
additional chances of storms. It`s not until late in the weekend
into early next week that a change in the pattern is possible.




315 PM CDT

Continued Hydrologic Outlook (ESF) for concerns of multiple
rounds of storms in an atmosphere that favors heavy rainfall, and
wet antecedent conditions particularly over the Des Plaines and
Fox River basins. Precipitable waters will be consistently above
1.60" for much of Wednesday night-Saturday night and so the
moisture will be present with fairly high confidence. The
uncertainty resides in placement and timing of storms, which given
the forecast weather pattern, is a particularly high uncertainty.
Nonetheless, computer models have indicated that the general
southern Wisconsin/northern Illinois region should see storms late
Wednesday night into early Thursday, and then possibly additional
multiple rounds Thursday night into Friday. These could yield
quite a gradient in rainfall amounts with much lower south (south
of I-88) to higher north.

The bottom line is that there are a lot of storm chances coming
up in an environment that can produce copious rainfall, with a
few periods of likely storms within that probable as the time
period draws nearer.



For the 06Z TAFs...

Two areas of activity are being watched this coming
east out of Iowa and the other dropping south from MN and WI. Both
are pretty vigorous at this time but the sense is that they will
trend weaker with the loss of daytime heating. Activity to the
west appears poised to reach the RFD area toward 03-04z but again
on a downward trend, so elected to carry VCTS for that area. The
Chicago area terminals appear to be more in the path of the
activity to the north, which we roughly estimate to reach the area
during the predawn hours. Again this is addressed with a VCTS
given the expected downward trends. Both areas of activity will be
closely monitored in case they end up being healthier and longer
lived, or faster moving, than presently anticipated.



311 PM CDT

Surface high over the eastern Great Lakes this afternoon, while
low pressure and front are situated from the western Great Lakes
back southwest through the upper Midwest. In this setup, southerly
winds of 15 to 25 kt are occurring with the strongest winds over
the northern half of the lake. As both of the features shift east
tonight, winds will turn more westerly while observing a
diminishing trend. Winds will then turn northerly on Wednesday, as
a front continues to move down the lake. Winds don`t get too high
with this wind shift though, as high pressure moves back across
the region into Thursday.






WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.